The processes used in water treatment depend largely on the nature of the raw water. Water supplies which feed industrial plants for the production of potable water for distribution and consumption, often contain unacceptably high levels of dissolved, dispersed or suspended organic compounds and materials. Most organic compounds and materials found in raw water supplies are natural organic matter (NOM). A fraction of the NOM in the raw water supply is represented by dissolved organic compounds which present particular difficulties. These organic compounds referred to as dissolved organic carbon (DOC), are one of the main causes of water discoloration. DOC often includes compounds such as humic and fulvic acids which are water-soluble at certain water pH levels. Humic and fulvic acids are not discrete organic compounds but mixtures of organic compounds formed by the degradation of plant residues.
The removal of DOC from water is necessary in order to provide high quality water suitable for distribution and consumption. A majority of the compounds and materials which constitute DOC are soluble and not readily separable from the water. The DOC present in raw water renders conventional treatment difficult and expensive.
The production of safe potable water from a raw water supply often requires treatment of water to make it aesthetically acceptable, as well as being safe to drink. The removal of suspended matter and DOC is an important aspect of this treatment.
The removal of DOC in water treatment plants can be facilitated by many processes including the use of membrane filtration processes like nanofiltration and reverse osmosis. Depending upon the composition of the raw water and the membrane itself such processes are able to remove up to and greater than 90% of DOC. However, apart from removing DOC, these processes are also able to remove salts contained within the raw water including NaCl. They are also used in desalination of seawater and brackish water. The end result of such processes is the collection of concentrated mixtures of DOC and salts which are rejected by the membrane and tends to foul them causing them to be less effective over time. As can be imagined this process produces a substantial amount of waste DOC and salt mixtures which are ultimately disposed of to land application or discharged into the ocean.
Of growing importance in water treatment technology is the use of ion-exchange resins, which can be used for removing DOC from raw water. Ion-exchange techniques conventionally involve passing water through a packed bed or column of ion-exchange resin. Target species (DOC) are removed by being adsorbed onto the ion-exchange resin. Ion-exchange resins can be used to remove up to 90% of the DOC in raw water.
Ion-exchange resins may also be used in conjunction with other methods of water purification. Sufficient resin may be added to remove a percentage of the DOC such that the cost of any subsequent treatment used to meet water quality objectives is minimized. For example, the use of ion-exchange resin for the removal of DOC can facilitate the reduction of the amount of coagulant required to achieve acceptable product water quality.
Ion-exchange resin may also aid in significantly reducing the capital and operating costs of membrane filtration.
In order to further minimize costs in water processing, ion-exchange resins are preferably recyclable and regenerable. Recyclable resins can be used multiple times without regeneration and continue to be effective in adsorbing DOC. Regenerable resins are capable of being treated to remove adsorbed DOC, and as such, these regenerated resins can be reintroduced into the treatment process.
Ion-exchange resins incorporating dispersed magnetic particles (magnetic ion-exchange resins) readily agglomerate due to the magnetic attractive forces between them. This property renders them particularly useful as recyclable resins as the agglomerated particles are more readily removable from the water. A particularly useful magnetic ion-exchange resin for the treatment of raw water is described in WO96/07675,the entire contents of which is incorporated herein by reference. The resin disclosed in this document has magnetic particles dispersed throughout the polymeric beads such that even when they become worn through repeated use, they retain their magnetic character. Ion-exchange beads of the type disclosed in this document are available from Orica Australia Pty. Ltd. under the trademark MIEX®.
WO 96/07615,the entire contents of which is incorporated herein by reference describes a process for removing DOC from water using an ion-exchange resin which then can be recycled and regenerated. This process is particularly useful in treating raw water with magnetic ion-exchange resin of the type described in WO96/07675.
The preferred ion-exchange resins disclosed in WO96/07675are magnetic ion-exchange resins which have, throughout their structure, cationic functional groups which provide suitable sites for the adsorption of DOC. These cationic functional groups possess negatively charged counter-ions which are capable of exchanging with the negatively charged DOC. Accordingly, the negatively charged DOC is removed from the raw water through exchange with the resin's negative counter ion. As a result of this process DOC becomes bound to the magnetic ion exchange and the function of the ion-exchange resin is reduced. For producing potable water for distribution and consumption it is particularly important to be able to regenerate the spent or DOC loaded magnetic ion-exchange resin in an efficient and cost effective manner.
WO 96/07615 discloses a process for regenerating magnetic ion-exchange resin by contacting it with brine (substantially a NaCl solution). The negatively charged DOC which is bound to the resin is removed through exchange with the regenerant salt negative counter ion. The by-product of this regeneration process, referred to as the “spent regenerant”, is primarily a mixture of removed DOC and excess brine. Like the waste products generated in the already discussed membrane processes the spent regenerants of this process are also disposed of to land application or discharged into the ocean.
These waste mixtures of salt and DOC are usually disposed of to land application when the water treatment is carried out in inland areas where ready access to the ocean is not available. It has been estimated that in the process described in WO96/07615,every million liters of raw water treated per day generates approximately 200-400 liters of spent regenerant depending on the raw water quality. This method for disposing of the spent regenerant can be environmentally unacceptable in many inland areas. In particular, the large concentrations of deposited NaCl which is produced as a by-product of the aforementioned processes cause degradation of soil quality. For instance, studies have attributed the high concentrations of sodium in the spent regenerant to an increase in soil salinity and water logging.